1. Field of the Invention
The disclosure relates to the field of audio processing. More particularly, the disclosure relates to acoustic echo cancellation and noise reduction in multiple microphone audio systems.
2. Description of Related Art
Mobile speech communication can be conducted under various environments. The microphones on the mobile device receive not only the desired speech, but also background noise. In many situations, background noise can be abundant. It reduces intelligibility of desired speech. Acoustical echo is another problem in mobile speech communications. Not only it reduces desired speech's intelligibility, it also distracts the far end talk and is very annoying. To improve intelligibility of desired speech, it is necessary to reduce background noise and acoustic echo without distorting the desired speech. Many echo and noise reduction methods have been developed.
Traditionally, noise suppression is achieved using only one microphone. One of such noise suppression methods uses spectral subtraction to suppress background noise. The method assumes that the background noise is short-term stationary, i.e. the noise statistics do not change in a short period regardless the activity of the desired speech. Noise statistics are estimated when a desired speech signal is absent and the noise estimates are used to suppress noise in the signal regardless of the activity of desired speech. Spectral subtraction estimates noise statistic and suppress noise in the frequency domain. Each frequency bin is processed independently. This method finds success in stationary noise reduction. However, it is not capable of reducing non-stationary noise.
Another single-microphone noise reduction method uses a directional microphone. Usually, uni-directional microphones are more expensive than omni-directional microphones. Also, uni-directional microphones may limit the way the speech communications devices are used since the mobile device may need to be placed properly to ensure its functionality.
Echo cancellation is typically achieved by de-correlating microphone signal from far end signal using adaptive filtering. Some aggressive echo cancellation algorithms reduce communication into half-duplex mode, where only one user talks at a time. In mobile speech communication, background noise and acoustic echo reduce intelligibility of desired speech. Therefore, it is desirable to reduce both background noise and echo without distorting desired speech.